1. Growing 33-d-old broiler fowl were exposed to air temperatures of 35 to 40 degrees C for a period of 4 h each day for 5 d. 2. The birds were fed on diets providing widely differing proportions and intakes of energy as protein, carbohydrate and fat. 3. Three lighting and feeding treatments were superimposed on the 4-h heat stress period. The treatments were: LF (control) lights and food throughout, NL (no light) extinguished throughout the hot period, NF (no food) food withdrawn 2 h before the hot period and replaced immediately after it. 4. Diet had no significant effect on rate of increase of rectal temperature (TR) during heat stress. 5. The NL and NF treatments significantly reduced the rate of increase of TR and the TR attained after 4 h of heat exposure.